Corrosion is one of the major problems associated with metallic structures, since all metals are known to exhibit a tendency to be oxidized. Cathodic protection is one way of protecting the metallic structures and preventing corrosion as a standard corrosion protection practice in many industries.
Cathodic protection can, in principle, be applied to any metallic structure in contact with an electrolyte. In practice, its main use is to protect steel structures buried in soil or immersed in water. The structures commonly protected include the exterior surfaces of pipelines, ship's hulls, jetties, foundation piling, steel sheet-piling, and offshore platforms. Cathodic protection is also used on the interior surfaces of water storage tanks and water circulating systems.
In cathodic protection, the metallic structure to be protected acts as the cathode and receives a direct current from the anode which is a consumable, semi-consumable or permanent anode. By cathodic protection, only direct current is entering the cathode surface to suppress the corrosion current from leaving the cathode surface but there is no formation of any protective coating.
The protective current density required for cathodic protection of the steel structure is predominantly determined by its surface condition and also the environment conditions it is exposed to. Bare or badly corroded steel surface requires higher protective current hence higher energy cost, while a well coated steel surface requires much less protective current. However, costs for applying the coating on the steel structure in submerged zone or in the buried zone is high and in general, the coating would not last till the end of the structure service life. Renewal of the coating during the service life of the structure is even more costly. For this reason, many steel structures are left uncoated and relying on the high protective cathodic current for corrosion protection.
Whether the submerged or buried steel structure is coated or bare, whenever there is stray or interference current, the steel structure immersed in the electrolyte would be subjected to the interference corrosion as the steel in contact with the electrolyte may be dissolved into Fe ions when DC current leaves the steel surface according to the following equation,Fe→Fe+++2e−
Yet cathodic protection has another limitation. The operating voltage of the cathodic protection anode is dependent on the electrolyte conductivity. When the steel structure is exposed to a low conductivity electrolyte such as river water or estuary water, a high driving voltage is required to drive out the protective current from the anode as the low conductivity water increases the resistance from the anode to the electrolyte (i.e. water). This results in high energy consumption of the cathodic protection system and difficulty in designing a suitable cathodic protection scheme.
In general, cathodic protection is a passive method of protection, it is unable to produce its own protective coating but relying on the externally applied protective coating to reduce its required protection current and yet it is still vulnerable to interference corrosion current and also requires a high voltage to operate under the low conductivity condition.
The cathodic protection method has been extensively used to protect the metallic structure by providing an anode which has a different potential compared to the metallic structure and corrodes preferentially. Two types of anode are available: sacrificial and impressed-current type. In addition to the drawbacks and shortcomings as mentioned above, the sacrificial anode and the impressed-current system have their respective limitations which have been well-known in the art.
Presently, there are also methods used for closed loop water treatment systems, which methods use the pulsating electromagnetic wave to treat the water for corrosion protection of the steel structures in water, instead of treating the steel structure directly. However, these methods are ineffective and impractical for open loop water systems such as jetty steel piles in the open sea.
Therefore, there exists a need for new apparatus and method that are capable of providing long-lasting protective coating for the surface of a metallic structure by treating the structure directly at low cost, irrespective of whether the metallic structure is buried in soil or immersed in water or whether the metallic structure is in a closed loop system or in an open system, however, allows an efficient control and prevention of corrosion of the structure.